The present invention relates to an assembly of variable capacitance in accordance with the introductory clause of Patent claim 1, as well as to a method of setting a predeterminable capacitance by application of this assembly.
The principle field of application of the present invention is within the domain of high-frequency technology, particularly in applications in communication technology. In this field, there is an ever-increasing demand for settable high-frequency capacitance levels at a maximum of quality achievable. So far, semiconductor components, so-called varactors, have been used to this end, which, however, reach Q-factor levels between 20 and 40 at maximum. Specifically for the application in mobile telephony or cellular phones, however, Q-factors of at least 100 are desirable. These high Q-factors in a resonant circuit can be achieved, at present, only with the use of micro-mechanical capacitors.
Capacitors manufactured by micro-engineering means are mechanically mobile elements that permit a variation of the distance or the overlapping or coverage degree of two capacitor plates for setting different capacitance levels. When such mobile elements are used a reaction of the component to exterior accelerations cannot be completely precluded, however. To this adds that in the high-frequency range at the frequencies from 0.8 to 2 GHz, which are specifically employed in cellular telephones and with the intended low attenuation levels, mainly metal materials are used as basic material for the mobile elements. Their specific density is definitely higher than the density in silicon, for instance, so that the mass of the mobile element is additionally increased by these materials. Low driving voltage levels and the resulting low driving forces for the mobile elements, in combination with the desired positioning distances of several 10 xcexcm, require comparatively soft suspending systems.
On the other hand, a settable capacitor for mobile application should be as stable as possible so that neither thermal nor mechanical influences from the outside cannot lead to a drift in capacitance. This resistance, particularly to accelerations, cannot be achieved with the afore-described properties of the known micro-mechanical assemblies.
The present invention is therefore based on the problem of providing an assembly of variable capacitance as well as a method of operating this assembly, which ensure a high stability of the respective capacitance set in resistance to outside influences.
The problem of the invention is solved by the assembly according to claim 1 as well as by the method according to claim 18. Expedient embodiments of the assembly as well as of the method are the subject matters of the dependent Claims.
In the inventive assembly, the variable capacitance is constituted by a variable coverage or a variable distance of at least one first and one second electrically conductive region. In the context of the present patent application, the term xe2x80x9ccoveragexe2x80x9d denotes an at least partial mutual overlapping or coverage of the two regions, seen in a viewing direction substantially orthogonal on or parallel to the surface of the substrate of the assembly.
The first electrically conductive region is configured here on or in the substrate while the second electrically conductive region is configured on or in an actuator element of a first micro-mechanical actuator. Both regions are preferably constituted by plane layers or plate-shaped elements and are substantially parallel to each other whenever the capacitance is set. However, this is not a definitely required prerequisite for the function of the assembly.
The first micro-mechanical actuator is so configured and disposed on the substrate that it is capable of performing a movement of the actuator element with the second region along the surface of the substrate at different positions relative to the first region, where the second region overlaps, at least partly, the first region. The first region is preferably disposed in or below the substrate surface and in parallel with the latter. The first region, however, may also extend in the form of a separate structure in a direction orthogonal on the substrate surface.
In the different positions which the actuator element may take, hence different distances and/or degrees of overlapping coverage prevail between the first and second regions, which gives rise to a different capacitance level. In accordance with the present invention, moreover holding means are provided on the substrate, which are capable of pulling or pushing the actuator element in the different positions towards the substrate or a mechanical stop on the substrate, and of holding it in this position. This holding function of the present assembly prevents a variation of the respective positions set and hence of the capacitance level set in the event of outside influences acting upon it.
The fixing of the actuator element relative to the substrate or the first region, respectively, can be ensured by both an electrostatic holding force and a further holding element producing a purely mechanical action upon the actuator element.
The holding means for the implementation of an electrostatic holding function can be achieved here in a very simple manner by the configuration of further electrically conductive regions on or in the actuator element and on or in the substrate, which are opposed to each other in the different positions to be set, so that the actuator element will be pulled towards the substrate surface due to the application of a differential voltage between these further electrically conductive regions. As a matter of fact, either an insulating layer must be configured over the additional electrically conductive region on the actuator element or on the substrate, or the respective regions are held by spacer or stops on the substrate or on the actuator element at a distance in order to avoid a short circuit. This applies also to the first and second electrically conductive regions that constitute the variable capacitance. These regions, too, must not come into direct contact with each other when the holding position is realised.
The holding means may also be implemented by a thermo-mechanical micro actuator. This micro actuator is so configured and disposed relative to the first micro-mechanical actuator that in response to a thermal excitation, it will be deflected in a substantially orthogonal direction on the surface of the substrate and that a first section of the actuator element of the first micro-mechanical actuator in the different positions to be set or that can be set up reaches up to a position underneath a second section of the thermo-mechanical actuatorxe2x80x94if the latter is in a deflected state. When the thermo-mechanical actuator is switched off the actuator element of the first micro-mechanical actuator is then clamped between the first section of the thermo-mechanical actuator and the substrate. Due to this clamping effect, a holding function can be expediently implemented, which does not require that energy be supplied during the holding function. For release of this holding position, the thermo-mechanical actuator is, in its turn, heated so that it will expand or will be deflected, respectively, and hence releases again the first micro-mechanical actuator.
In addition, the respective sections of the two actuators, which are superimposed on each other, may present corresponding structures that permit a mutual engagement or mutual hooking at the respective holding positions. This ensures a particularly stable holding position.
The first micro-mechanical actuator may be configured, for instance, as electrostatic or thermo-mechanical actuator. With appropriate suitability, micro actuators operating on other driving principles can, equally be employed, of course. Electrostatic micro actuators are, however, particularly well suitable for the application in a network-independent device such as a cellular telephone, due to their low energy consumption. Moreover, an electrostatic operation permits high-speed switching in the range of 100 xcexcs.
The structure of suitable micro-mechanical actuators is common to those skilled in the art, which are appropriate for the application in the inventive assembly. The common methods of microstructure technology can be applied for the manufacture of such micro actuators and of the present assembly. For the manufacture of the variable capacitors, specifically those methods come into question, which operate either on the basis of polysilicon or on methods for the realisation of the mechanical components proper. Both manufacturing techniques are part of the field of superficial micro mechanics.
In operation of the inventive assembly, the first micro-mechanical actuator is deflected in the envisaged manner and when the desired position or capacitance, respectively, is reached the holding means are controlled to maintain this position. The respective actual position of the actuator element may preferably be realised by measuring an appropriate reference capacitance (or differential capacitance, respectively) on the substrate. The measurement of this reference capacitance during the deflection enables a very high resolution or a very precise setting of the variable capacitance. The reference capacitance can be constituted by additional small capacitors that may be adjacent to the variable capacitance proper (high-frequency capacitance). These additional capacitors may be employed as position-sensitive sensors. The actual target capacitance, however, is reached only when the holding function is activated because this holding function varies the distance between the first and second electrically conductive regions again, at least in the preferred embodiment of the assembly. In the operation of the present assembly in a closed-loop arrangement, with integration of the additional reference capacitor or reference capacitors, respectively, hence the desired capacitance of the assembly can be set with a very high precision.
In an expedient operating mode, the first micro-mechanical actuator is periodically controlled at its natural frequency. This can be realised in a simple manner particularly when this actuator is driven in an electrostatic manner. The second electrically conductive region periodically sweeps over the first electrically conductive region and, when the desired position is reached, it is fixed by controlling the holding means so that the desired capacitance value is maintained. Due to the periodic control, wide deflections of the actuator element can be reached with comparatively low driving voltages. Moreover, in this variant of the operating mode of the assembly, the above-described closed-loop assembly is preferably employed for determining the position.
The operating range of the inventive assembly can be extended by the provision that a number of additional switchable capacitors are disposed on the substrate. These switchable complementary capacitors consist, for instance, of invariable capacitors that can be additionally connected via appropriate high-frequency switches of the variable capacitance. The additional switchable discrete capacitance elements may also be implemented in their capacitors in a binary arrangement. Due to the combination of such a capacitor network with the variable capacitance proper, it is possible to set a wide range of capacitance levels.
In another alternative embodiment, several ones among the inventive assemblies are disposed on a substrate whose variable capacitance elements are connected in parallel.